Propeller blade



E. MARTIN PROPELLER BLADE Filed Dec. 14, 1940 ATTORNEY suppressingvibration of. the

Patented Jan. 25, 1944 PROPELLER BLADE Erle Martin, West Hartford,Conn., assignor to United Aircraft Conn.,

Corporation, East Hartford, a corporation of Delaware ApplicationDecember 14, 1940, serial No. 370,171

(ol. 17o-159) 14 Claims.

This invention relates to improvements in propeller blades and haskparticular reference to an improved metal blade foran aeronauticalpropeller.

An object of the invention resides in the provision of an improvedpropeller blade having f energy dissipating means incorporated thereinfor blade during propeller operation.

A further object resides` in the provision, in a metal propeller bladeof the character indicated, of vibration suppressing means carried bythe outer portion of the blade without materially weakening the blade orchanging its weight.

Other objects and advantageswill be more partlcularly pointed outhereinafter or will become apparent as the description proceeds.

In the accompanying drawing, in which like reference numerals are usedto designate similar parts throughout, there is illustrated a suitablestructural embodiment for the purpose of disclosing the invention. Thedrawing, however, is for the purpose of illustration only and is not tobe taken as limiting or restricting the invention since it will beapparent to those skilled in the art that various changes in theillustrated construction may be resorted to without in any way exceedingthe scope of the invention.

In the accompanying drawing,

Fig. 1 is an elevational view of a propeller blade showing theincorporation therein of vibration suppressing means constructedaccording to the invention.

Fig. 2 is an elevational view on an enlarged scale of the outer or tip.portion of the blade lllustrated in Fig. 1.

Fig. 3 is a cross-sectional view on an enlarged scale taken on the line3-3 of Fig. 2.

Fig. 4 is a cross-sectional view of a fragmentary portion of thepropeller blade tip showing a rivet aperture .extending therethrough. f

Fig. 5 is a longitudinal sectional view of a tubular rivet.

Fig. 6 is an elevational view of the tubular rivet, and

Fig. 7 is a sectional view of a fragmentary portion of the propellerblade tip similar to' Fig. 4 showing the rivet of Fig. 5 and the plug ofFig. 6 assembled in operative position therein.

Referring to the drawing in detail, the numeral I0 generally indicatesthe metal propeller blade secured at its root or base end in a-propeller hub generally indicated at l2. The blade maybe se- A cured inthe hub for turning movements about the longitudinal axis of the bladeto change the a plug for :filling` propeller pitch in a manner wellknown to the art.

It is a well known fact that propellers are subject to severevibrationconditions arising from various sources such as the vibratory componentsof engine torque, eccentric rotation of the propeller shaft and dynamicand aerodynamic unbalance of the propeller itself. These vibrationaleffects are particularly severe in the case of anaeronautical propellerbecause of the great length of the )propeller blades in proportion totheir weight and Width and the high speed at which such propellers arecustomarily driven and such vibrational conditions may becomeparticularly Vsevere 'in the case of all metal propeller blades sincethe metal does not have as great internal damping characteristics asdoes the material of a blade formed of wood or other non-metallicmaterial. It has been found that the outer or tip portion of metalblades usually have a greater amplitude of vibration induced movementthan Y any other portion of the blade and that frequently a node existsbetween the extreme outer or tip portion of the blade and the innerportion. Such a nde usually occurs in the outer-half of the blade wherethe section of the blade is relatively thin and the concentration ofstresses about the location of the node has frequently led to fatiguefailure of the propeller blade.

In carrying out the invention, the outer part of theblade is firstdivided or split by means of a saw cut disposed substantially in a planeincluding the neutral surface of the blade, that is, the surfaceincluding vthose points Within the blade which remain free of strainwhen the blade is bent. The plane Vof the saw cut does not exactlyfollow the neutral surface since it is more convenient from amanufacturing standpoint to have the saw cut straight and it has beenfound entirely satisfactory to-provide a straight saw cut through thewidth of the blade with substantially equal amounts "of metal onopposite sides of the cut, as is particularly shown in Fig. 3. While theextent of the saw cut relative to the length of the blade will dependupon the vibrationalcharacteristics of the particular blade I somewhatgreater tharrone-,third of the total blade length. y

After the saw cut has beenf made, a sheet of some suitable materialhaving relatively high externa! friction characteristics and acomparatively high internal hysteresis is inserted in the cut to fillthe same. 'I'he thickness of the saw cut and of the inserted materialwill also depend upon the vibration characteristics of the particularblade shape and the material of which the blade is made and upon thedamping characteristics of the inserted material. It has been found thata relatively thin sheet of material such as Micarta or Formica is'adequate because of the relatively high external friction when incontact with the surfaces of the metal blade parts and its high internalhysteresis effect.

After the sheet of material I4 has been inserted in the saw cut providedin the outer portion of the blade, a plurality of tubular rivets I 6 areinserted through apertures provided through the blade and the dampingmaterial. These apertures i8 are preferably provided with-flared ends asindicated at 20 and 22 and the tubular rivets I6 may have one endpre-flared, as indicated at 24; to nt the flare of one end, of anaperture. After the tubular rivet has been inserted through the apertureits straight end is flared outwardly, as indicated by broken lines `inFig. 5, to llt the ilare at the opposite end of the: aperture and clampthe two parts of the propeller blade on 4opposite sides of the 'materialMinto firm contact with the sheet of inserted material. The rivets I8are preferably made of some relatively strong and consequentlycomparatively heavy material, such as mild steel, and are made tubularin order to avoid an unnecessary increase in the Hweight of the outerblade portion. .After the tubular rivets are in place they may be illledwith suitable plugs 26 of light weight material such as aluminum whichplugs may be provided with flared or expanded end portions to fl1l"thecontact with the inserted material when the blade tip bends under thestress of aerodynamic or vibration induced forces.

'I'he above described construction provides an arrangement in which theouter portion of the propeller blade constitutes a composite cantileverbeam. Since the friction material I4 is disposed substantially along theneutral plane or surface of the original blade form this material doesnot tend to move as the blade is bent. The two portions of the blade onopposite sides of the saw cut will bend, each about its own neutralplane or axis with the result that one of the surfaces in*v contact withthe friction material lwill'be'in a stateof compression and thealternative surface will be in a state of tension.' Since both partsseparated by the saw Vcut are held together at the -bottom of the sawcut there will be a slight relative motion between the opposite faces ofthe saw cut whenever the blade is bent. This relative motion will createfriction between the inserted friction material and the/contactingsurfaces of the outer blade portion which friction will damp outvibrational forces tending to bend 'the blade by dissipating the energyof these forces. The internal hysteresis of the friction material addsto the frictlonal damping effect by dissipating a portionof the energyof the blade bending forces in internal friction of the material. Whilea suitable constructional embodiment has been illustrated in theaccompanying drawing and hereinabove described for the purpose ofdisclosing the invention, it is to be understood that the invention isnot limited to the particular embodiment so illustrated and described,but that such changes in the size, shape and arrangement of the variousparts may be resorted to as come within the scope of the sub-joinedclaims. Having now described the invention so that others skilled in theart may clearly understand the same, what it is desired to secure byLetters Patent is as follows: v`

' 1. A flexible metal propeller blade having a laminated outer portionincluding the blade tip and a non laminated inner portion including theblade shank and having major dimensions of length from the tip end tothe shank end thereof, breadth from the leading edge to the trailingedge thereof, and thickness between the face sur.

lfaces thereof, the laminated outer portion comprising separate closelyadjacent blade parts in- .25 tegral with the blade shank and whose majorsurfaces lextendvlengthwise of said blade, solid sheet materialsubstantially coextensive with the made parts of sindA iaminated'pornonand hem under compression between and in frictional con- 30 tact withsaid parts and movable, upon blade vibration, relative to the blade partsurfaces with which it is in contact and when so moved dissipatinsenergy through slidingsurface friction.

2. In a flexible metal propeller blade having a laminatedportion and anonlaminated portion and having major dimensions of length from theshank end to the tip end thereof, breadth from the leading edge to thetrailing edge thereof and thickness between the-face surfaces thereof,said 4'() laminated portion including blade laminations each having asurface'extending lengthwise of said blade from a location adjacent thetip end ofthe blade-to the nonlaminated portion, alternate laminationsintegral with the metal of said blade, a lamina of a material differentfrom the metal of said blade secured between each two adjacent blademetal laminae, and means mainv taining said material under compressionbetween and in frictional contact with said blade metal laminae, todissipate energy through sliding surface friction upon relative movementof adjacent laminae during blade vibration. y

3. In a flexible solid metal propeller blade of airfoil contour having aneutral plane within said liliv contour extendingalong the major axis ofthe blade and chordwise of the blade from adjacent the leading edge toadjacent the trailing edge thereof, the tip of said blade having a slotformed therein and defined by maior surfaces substantially'parallel withsaidv neutral plane and-ex tending inwardly from the blade tip end andterlminating less than two-thirds ofthe blade length from said tip, aninsert of sheet material held under compression in said slot and infrictional 05..engagement with the tip end portion of said blade, saidsheet material, uponl blade vibration,

being movable relative to the tip= end portion of the blade with whichit is in contact and when so moved dissipating energy through slidingsurface friction. f

4. In a flexible metal propeller blade having a thickness between therface surfaces thereof said laminated portion including integralextensions of said nonlaminated portion, vibration suppressing means insaid laminated portion comprising sheet material extending less thantwothirds of the blade length toward said blade shank f from adjacentthe tip endof the blade and having its major surfaces extending in thedirection of .blade length and breadth said material being substantiallycoextensive with said integral extensions and located intermediate theblade thickness and assembled in frictional contact with said integralextensions of said nonlaminated portion, to dissipate energy throughslidtact and when so moved dissipating energy through sliding surfacefriction.

8. A metal propeller blade having a nonlaminated shank portion and a tipportion comprising two parts integrally joined at one end to each otherandA to said shank portion and separated -by a narrow slot disposed in aplane extending along a chord of the blade and along the major 1 axisthereof and extending from adjacent the between the face surfacesthereof, vibration suppressing means comprising, blade laminations eachhaving a major surface thereof extending lengthwise of said blade eachlamination extend` ing only from a location greater than one-third theblade'length from the shank end to adjacent the tip end thereof, saidblade tip being slotted to form said blade laminations constituted bythemetal of said blade, and a lamination of solid sheet material -securedbetween and in frictional contact with each two adjacent blade metallaminations and movable, upon blade vibration, relative to thelaminations with which it is in contact and when so moved dissipatingenergy through sliding surface friction.

6. In a exibie metal propeller blade having major dimensions of lengthfrom the shank end to thel tip end thereof, breadth from the leadingedge to the trailing edge thereof and thickness between the facesurfacesthereof. vibration suppressing means comprising, bladelaminations each having a surface extending lengthwise of said bladefrom adjacent the tip end and terminating less than two-thirds of theblade length from said tip and extending across the breadth of saidblade between the leading edge and the leading edge/of the bladetip'portion to adjacent the trailing edge thereof and inwardly from theblade tip to the nonlaminated shank portion, solid sheet material insaid cut substantially filling the same and constituting with saidy twoparts blade tip laminations, and means for holding said laminations inkfrictional engagement, so that relative movement of said laminations,upon blade vibration, will dissipate energy through sliding surfacefriction. f 1

9. A flexible metal propeller blade having `a laminated outer portionincluding the blade tip and a nonlaminated inner portion including theblade shank and having major dimensions of length from the tip end tothe shank end thereof, breadth from the leading edge to the trailingedge thereof, and thickness between the face surfaces thereof, thelaminated outer portion comprising separate closely adjacent blade partsintegral with the bladeshank and whose major trailing edge thereof,alternate laminations belng metal and having a portion integral witheach other and said blade, and a lamina of a material different from themetal of said blades secured between each two adjacent blade metallaminae and movable, upon blade vibration, relative to the blade metallaminae with which it is in contact and when so moved dissipating energythrough sliding surface friction.

'7. In a flexible solid metal propeller blade hav- -ing major dimensionsof length fromthe shank end to the tip end thereof, breadth from theleading edge to the trailing edge thereof and thickness between the facesurfaces thereof, a split in the tip portion of the blade extending fromthe blade tip end through said tip portion in the direction of the bladelength to a location less than two-thirds of the blade length from theblade tip and through said tip portion in the direction of blade breadthintermediate the thicknessof the blade tip portion separating saidtipportion into two parts integrally united at their inner ends witheach other and with the remainder of said blade, solid sheet materialheld under compression between and in frictional contact with said partsand movable, upon blade vibration, relative to the blade part surfaceswith which it is in consurfaces yextend lengthwise of said blade, solidsheet material having different internal friction characteristics thanthat of the material of said blade held under compression between and infrictional contact with said parts and movable,

-upon blade vibration, relative to the blade part surfaces with which itis in contact and when so moved dissipating energy through slidingsurface friction. v

10. In a flexible metal propeller blade having a laminated portion and anonlaminated portion and having major dimensions of length from theshank end to the tip end thereof, breadth from the leading edge to thetrailing edge thereof and thickness between the face surfaces thereof,said laminated portion including blade laminations each having a surfacesaid blade from a location adjacent the tip end of the blade to thenonlaminated portion, alternate laminations being constitutedby themetal of said blade and integral therewith, a lamina of amaterialdifferent from the metal of said blade and having differentinternal friction characteristics than that of thematerial of said bladesecured between each two adjacentbiade metal laminae, and meansmaintaining said material under compression between said blade metallaminae, to dissipate energy by sliding surface friction upon relativemovement of adjacent laminate upon blade vibration.

11. A metal propeller blade having a tip portion comprising two partsintegrally joined at one end and separated by a narrow slot disposed ina plane extending along a chord of the blade and along the major axisthereof and extendingfrom adjacent the leading edge of the" blade tipportion to adjacent the trailing edge thereof and inwardly from theblade tip to a locationless than two-thirds of the blade length from theblade tip, an .insert of a material, having different internal frictioncharacteristics than that of the material of said blade in said cutsubstantially filling the same and constituting with said two partsblade tip laminations, and means for holding said laminations invfrictional engagement so that relative movement of said blade and saidinsert inciextending lengthwise of dent to blade vibration willdissipate energy through sliding surface friction.

12. A flexible metal propeller blade having a laminated outer portionincluding the blade tip and a nonlaminated inner portion including theblade shank and having major dimensions of length from the tip end tothe shank end thereof,

breadth from the leading edge to the trailing edge thereof, andthickness between the face surfaces thereof, the laminated outer portioncomprising separate closely adjacent blade parts integral with the bladeshank and whose major surfaces extend lengthwise of said blade, solidsheet material having energy absorbing internal friction characteristicsheld under compressions between and in frictional contact with saidparts and movable by said blade parts and relative to the blade partsurfaces with which it is in contact and when so moved by bladevibration, dissipating energy through sliding surface friction andthrough internal friction of said solid sheet material.

of said blade and integral therewith, a lamina of a material differentfrom the metal of said blade and having energy absorbing internalfrictional characteristics secured between each two adjacent blade metallaminae to absorb energy through internal friction upon movement of saidmaterial by said blade under blade vibration, and means maintaining saidmaterial under compression between said blade metal laminae, to

dissipate energy by sliding vsurface friction upon relative movement ofadjacent laminae.

14. A metal propeller blade having a nonlaminated shank portion and atip portion comprising two parts integrally joined at one end to eachother and to said shank portion and separated by a narrow'slot disposedin a plane extending along a chord of the blade and along the major axisthereof and extending from adjacent the leading edge of the blade .tipportion to adjacent the trailing edge thereof and inwardly from theblade tip to the nonlaminated shank portion, an insert of solid sheetmaterial having energy absorbing internal friction characteristics, insaid cut substantially filling the same and constituting with said twoparts blade tip laminations, and means for holding said laminations infrictional engagement, so that relative movement of said blade and saidinsert incident to blade vibration will dissipate energy through slidingsurface friction and movement of said insert by said blade will absorbenergy through internal friction of said insert.

ERLE MARTIN.

